DEFORESTATION AROUND THE WORLD - India Environment Portal

Landslides Caused Deforestation
the surface in the curves are partially due to the low viscosity of this layer and partially due
to the uneven surface loading in the down-slope direction of the borehole. At the time of this
study, measurements are available only up to March 27, 2008.
The displacement curve is assessed for January 1, 2010 based on the climatological
precipitation over the region (see Fig. 5). The basal sliding attributed to the Wenchuan
earthquake of 2008 also was taken into account. Compared with the initial measurements,
the shallow level displacements are as large as 18 mm. Based on our analysis of the effects of
the Wenchuan earthquake on the YC creeping slope, the creep soon will accelerate. The
Wenchuan earthquake reduced the natural creeping period by at least five years. For a
crevasse near BH06, the surface crack enlarged from 10 to 18 cm. If the crack geometry is
assumed to be constant, the crack depth almost doubles. Estimated depth changes were
made of other cracks. The changes in its natural sliding cycle are obtained by comparing its
creeping speed under current conditions with the pre-quake conditions. For example, after a
major slope adjustment, say in 1998, it is assumed that there are no cracks due to strain. In
2008, before the earthquake, the cracks are already monitored. They all are located at model
locations with large strain rates. The natural cycle is not difficult to estimate; 1mm/day is
the critical value for next major sliding event. Even if there is little change in the
precipitation morphology, over the next ten years there likely will be significant slope
movements. However, if there is an intense rainstorm with over 150 mm/day at any time in
the future period, then sliding becomes imminent.
Three historical landslides (1960, 1980 and 1997) reported in the YC study area are separated
by ~20 years (Fig. 6). In 1960 and 1997, high annual precipitation values of 1819 and 1771
mm, respectively, were recorded. However, 1980 was relatively dry with 1200 mm total
precipitation compared to 1600 and 1360 mm respectively for 1979 and 1981. Examination of
the daily precipitation series from 1979 and 1980 indicates that in 1979 over 90% of the
annual precipitation occurred in the latter half of the year, with no significant precipitation
before June. Although the total precipitation for 1980 is small, the heavy precipitation in
January 1980 immediately following the previous year’s precipitation events formed an
extended wet period. There was a significant precipitation event in January, reaching a rate
of 58.3 mm/day that lasted 4.7 hours on January 15. As a result, the deep soil moisture (0.23
volume per total volume) remained relatively high for the remaining several months. What
triggered the landslide was a very heavy precipitation event of a 100-year recurrence
frequency, with a single day precipitation of 230 mm on June 11th.
For storm-triggered landslides, those precipitation events separated by less than two dry
days can be considered as one single ‘super’ rain event. Thus, unlike many previous studies
(e.g., O’Gorman and Schneider 2009), which count daily precipitation one day at a time
(traditional precipitation analysis), we count those extended super-rain-events, defined as a
somewhat continuous rainfall period nowhere separated by more than two consecutive dry
days (rain-event analysis, Ren et al. 2011a). Figure 6 compares two methods of rainfall
analyses. Using a traditional analysis, the 1997 slide does not correspond any significant
daily rainfall event (>100 mm/day). However, following Ren et al. (2011), all major
landslides (red arrows in Fig. 6) are triggered by rain events with higher rainfall totals. For
the YC, heavy precipitation is both an enhancement factor, and a determining factor, in
triggering landslides. However, current observational data are less than 2 years long, which
is far too short to unambiguously resolve this hypothesis. SEGMENT-landslide simulations
indicate that, under long term mean soil moisture conditions, the creeping will achieve a
near-surface movement rate of 0.1 mm/day near the head of the slope, sometime within the
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